The concept of condition monitoring is based on a routine or permanent detection of the condition of a machine. The purpose of condition monitoring is on the one hand to increase safety and on the other to increase efficiency. During the condition monitoring of a system or the components thereof, the behaviour of the machine structure is the signal source. A change occurring in the characteristics of the machine structure caused, for example, by the wear of components, is an event to be detected by the machine monitoring procedure. Monitoring takes place by the measurement and analysis of significant physical variables, such as oscillations, temperatures, etc. Condition monitoring consists of a plurality of sub-steps:                detection of the condition comprises the measurement and documentation of system parameters which reflect the current condition of the respective system or components thereof.        Comparison of the condition is the comparison between the actual condition and a predetermined reference value. This reference value can be a desired value to be observed as well as a limiting value which is not to be exceeded.        Diagnosis: based on the results of the condition comparison, it is possible for errors which may be present to be localised as early as possible and for the causes thereof to be determined in order to be able to plan necessary maintenance measures in good time.        
Although the present invention and the problem it addresses can be applied in principle to any methods for condition monitoring, they will be described in the following with respect to a method and a device for condition monitoring for use in an aircraft or spacecraft. In particular, the present invention will be described in the following with respect to condition monitoring on the basis of oscillations measured on a component or system of an aircraft.
DE 10 2006 031 009 B4 describes a method and a device for monitoring the condition of structural components of an aircraft. The condition monitoring described therein is based on the optical examination of structural components and on the computer-assisted comparison of the recorded image with a reference image. It is thereby possible for the appearance of the structural components to be monitored. It is not possible to monitor the function of systems and components of an aircraft using this type of optical condition monitoring.
In the case of aircraft, particularly passenger aircraft, for safety reasons alone it is necessary to constantly observe fixedly predetermined maintenance intervals, within which aircraft components and systems have to be examined, maintained and repaired. These intervals are fixed to ensure that the respective components and systems are adequately safe and capable of operating until the next maintenance interval. For this reason, the corresponding maintenance intervals are selected to be suitably generous, typically with a time buffer.
However, it is a problem that with this approach, systems of an aircraft are sometimes also maintained although, for example, they are still fully operational and in fact there is no need for them to be maintained. Thus, it is easily understood that as a result, due to the unnecessary maintenance which is still carried out because the actual conditions are unknown, additional costs are incurred.
In addition, for example in the case of passenger aircraft, the periods of time allocated for maintenance work are very short. This work is usually carried out during the immobilisation times of the aircraft between the arrival of an aircraft at the terminal and the departure from the terminal, for which there is generally only a short period of time available of about 90 minutes. Therefore, in this respect, it is desirable to collect relevant information in advance about the condition of a system or component in the aircraft.
The air conditioning, which is generally also known as HVAC (heat, ventilating and air conditioning) is particularly significant in an aircraft in respect of functionality and of passenger comfort. On the one hand, the air conditioning is to supply the passengers in an aircraft with fresh, temperature-controlled air and to cool the electronic systems. For this purpose, the air-conditioning system uses recirculation fans and filters to be able to circulate the air in the cabin and to clean it. However, the filters which are used can become blocked over time, corresponding supply lines can become soiled and the fan can become faulty, for example, due to the electric motors failing or to the blades of the fan breaking, for example. It is a problem that it is very difficult to predict the blocking of the filters in particular, because this always depends on the surroundings of the respective filters and on the region of use. Therefore, this means that these filters are checked very often in situ, which involves additional examination times during the immobilisation times of the aircraft.
This is a situation which is understandably to be avoided.